Turbo Actuator Theory

[Robit28]

WARNING: LONG POST!

Hi All, I have just aquired my new 12.5psi actuator to try and cure my surging of boost pressure.

I have done some research on the forums and found that people have been told they would need a retune after the fitment of such a mod. I have been thinking about it and would like to know if my theory is correct.

From what I have learnt over the years, an electronic boost control system works by bleeding off air from the actuator vacuum line to keep a consistant boost level. It does this in a closed loop operation. When a prescribed boost level is reached, the solenoid closes (the solenoid has an open or shut state), the closing of the solenoid then rises the air pressure that the actuator 'sees' and the wastegate starts to open, therefore bypassing exhaust gasses from the turbo.

The turbo then slows down and boost starts to drop, this drop in boost is then sensed by the map sensor and the ECU then opens the solenoid to increase boost again as the prescribed boost pressures have not been met. Once the boost pressure is met once again, the cycle continues. This all happens continuously and very quickly.

I am assuming that the XR6T has a very simular setup to this closed loop operation. I beleive the ECU has a 'Boost Map' that can be mapped to RPM amoungst other variables. Now lets say that with my standard actuator, my boost map is programmed as below (this is just an example):

RPM---PSI

2500---14

3000---15

3500---16

4000---16

4500---16

5000---14

The computer will cycle the solenoid in order to keep these boost levels at these RPM's throughout the rev range. Now I know that other factors will determine boost pressures such as load, TPS etc but I will use RPM as an example.

Now let's say my spring pressure cannot hold the wastegate shut and boost cannot be kept stable, as with a standard spring at high boost. The 'actual' pressures may look something like this:

RPM---PSI

2500---14

3000---15

3500---16

4000---14

4500---13

5000---11

This is simular to what my car is experiencing at the moment with a weak actuator spring.

You can see that although the solenoid will be trying hard (high duty cycle) to bleed air off, no matter how much air is bled, the spring is too weak to hold the wastegate shut and therefore boost drops. Now all this time the ECU is telling the solenoid to stay open as long as it can as the prescribed boost pressure has not been met. This then raises the 'duty cycle' of the solenoid to is maximum and no more boost can be gained from the turbo. (My duty cycle maxes at 98% at the moment)

This is where my question begins. Let's say I now install a new actuator and it has a spring pressure of 12.5psi (which my new actuator does).

What will happen? This is how I would foresee it:

Your minimum spring pressure will always be your minimum boost level as the solenoid will just stay shut and the turbo will then be fully controlled by the spring pressure, so in my case, 12.5psi will be the minimum boost I could ever run. (Without modifications). Seeing as the minimum boost set in our example map is 14psi, that is fine for the example car.

Now with the new spring in, wouldn't the ECU controll the solenoid differently to adjust for the new spring like so:

When the prescribed boost is reached, the solenoid will shut therefore opening the wastegate to 'bleed' off boost pressure. Seeing as the spring is much stronger, this will happen faster and the duty cycle of the solenoid will be much less (it won't have to stay open for as long). After boost drops yet again, the ECU senses the drop and opens the solenoid again so air can be bled from the actuator, closing the wastegate to increase boost, the cycle starts again.

With the new spring, the closed loop operation still functions the same, only the timing of the solenoid will be different (lower duty cycle) as it doesn't have to bleed off so much air to keep the boost up.

So if the system is closed loop and operates in this manner, would there really be any need for a retune? The ECU would control the solenoid as it would normally, only adjusting the duty cycle of the solenoid as need be.

Sorry for such a long post, just thought you guys would be interested in my theory.

Any questions or corrections go right ahead. I would love to hear from tuners on this theory also to see what they think.

Rob.

Oi, you still awake!

[Paradoxx]

Hi Rob,

Very interesting what you have written here, I havent really considered how the whole balancing act goes with the wastegate actuator before. If you have upgraded the actuator spring I am assuming you have simply replaced the entire actuator assembly? Does the actuator come at a set value (ie yours being "set" to 12.5psi)? What is the stock one "set" at?

Looking at my graph it shows I hit boost up to about 14psi then it bleeds down to 10psi... I just assumed this was the solenoid bleeding the boost off and performing it's balancing act as such. If you upgrade the actuator does this mean I would not be able to bleed off to 10psi as a set boost level?

Got me thinking... good post

[Ninka]

I'll be intersted in this as well, as I am considering changing the standard actuator to one with a higher opening pressure.

I'm chasing high kw numbers, and expect poost to be reasonably high when my engine is complete, and I'm wondering if the standard actuator is up to the job.

Anybody have any experience?

Any suggestions where to get a good replacement actuator?

[Robit28]

Hi XXXR6T,

The entire assembly needs to be replaced as the actuator is a sealed unit.

All actuators have a set spring rate, this will be the boost level needed to open the wastegate if NO air is bled from the actuator. This will also be your 'minimum' boost level.

I beleive the XR6T's spring rate to be around 4-5psi standard. This will allow for minimum boost and allow the ECU to control the solenoid to give you 5-6psi of boost.

You can see now why turning up the boost to 14psi or more can become a problem!

You should probably buy a spring pressure suited to the minimum boost pressures you will be using. For example, if you want 10psi to be able to run, use a 10psi or a 9psi spring rate.

Hope that answers your question????

Rob.

[Robit28]

Hi Ninka, I bought mine from Advanced Turbo Performance in Melbourne (03 9569 4764). Cost was around the $220 mark with an actuator arm to suit the XR6T.

Very heavy duty looking unit.

I called a couple of well known tuners up and they recommended to go stright to ATP.

You can get them in a variety of spring rates, just let them know what you want. (Mine is 12.5psi)

Rob.

[Paradoxx]

Yes it does, thanks

So I guess you are asking does the ECU have to respond differently to a different spring pressure? I assume if you drop below your new "minimum" boost level the ECU would be trying to compensate for this and when it cant bleed enough boost you will have an overboost situation and will loose timing in a big way, giving you a surging power.

I have had a brief look at the tuning software and it does set boost levels vs rpm, but also has correction tables for other situations. I would say you will need a retune for sure, at least a look at the tables to see if your boost levels will drop below the spring pressure.

[Robit28]

I am wondering whether tuners keep their lower RPM vs BOOST maps quite high to stop overboosting.

For example:

Your turbo will only produce as much boost as it can at a certain amount of rpm's and load. (the turbo won't produce 20psi of boost at 2000rpm).

So it would be safe in saying that a turbo boost map should probably be set at say 10psi minimum up to a certain RPM where it will start to climb. So your map may look a bit like this:

RPM---PSI

1500--10

1750--10

2000--10

2250--10

2500--12

2750--12

3000--14

I think I have seen a XR6T boost map and all the 'lower' values are set to a high boost level, or the minimum boost level, eg 12psi.

That would stop your car from entering 'limp' mode if slight overboost was seen at lower RPM's.

Just a thought, good point though. If your boost map was set to say 8psi and you had a 12psi spring, you could almost guarantee 'limp' mode status.

Rob.

[Ninka]

The risk you run is that if the actuator is commanding more boost than the tune, then the car will reduce boost by other means, such as retarding ignition or cams.

The issue being that you are then having a mechanical boost controller at lower boost levels, and these need to match what the PCM is expecting.

Yes the standard actuator is a 3-4 psi unit, as I tried this in the early days, just to see what would happen if the solenoid valve was bypassed.

[GOLDIE]

The way the xr6t system is set up you will need a retune as the computor control the boost solinord on a duty cycle system and the % of duty cycle will need to be reset to the different actuator.

I fitted a new one on my car and with out a retune it would over boost all the time.

If you do not want to blow your motor do not do things by half have it retuned.

Ian

[Paradoxx]

I think you should look at it more from a duty cycle point of view, as this is how it would set the boost... in my previous car it was set as such...

1000 - 0%

1500 - 0%

2000 - 0%

2500 - 15%

3000 - 65%

3500 - 70%

4000 - 50%

4500 - 40%

5000 - 30%

See what I mean? As RPM increases the boost level will have to drop as more air is being fed (volume) at lower pressure... at the start the wastegate is closed to build pressure quickly, but is opened at the set level to control boost to the desired level, then dropped. If you "need" to drop your boost lower than 12.5psi and you are at 98% duty cycle then there's nothing you can do and you get an overboost condition. The ECU may already be set to drop your boost to say 10psi at higher rpm, so adding the new actuator it can't get it to 10psi and will fault...

I think we are on the same track but... I would get those maps looked at before you give it some at high rpm.

EDIT:

Also, aswell as the boost maps there will be a "boost limit" which will be set to control the max boost at a given RPM, so if say at 4000 rpm the "max" boost is set to 11psi and your actuator is giving you 12.5 then it will be overboosting and will die. My previous car's ECU was a little more primative and only had a max boost level set overall (one value) which protected the motor at its highest boost point. I would assume the falcon ecu has boost limits set throughout the revv range.

Edited 05/05/06 03:08 AM by XXXR6T